1. Technical Field
The present disclosure relates to an electrostatic micromotor with stator and rotor in contact, in particular for atomic-level storage systems (generally known as “probe-storage systems”), to which the ensuing treatment will make reference without implying any loss of generality.
2. Description of the Related Art
As is known, storage systems that exploit a technology based on magnetism, such as, for example, hard disks, suffer from important limitations as regards the increase in the data-storage capacity and the read/write speed, and the decrease in their dimensions. In particular, a physical limit exists, the so-called “superparamagnetic limit”, that hinders the reduction in the dimensions of the magnetic-storage domains below a critical threshold, if the risk of losing the stored information is to be avoided.
In the last few years, alternative storage systems have consequently been proposed, amongst which the so-called “probe-storage systems” have assumed particular importance. These systems enable high data-storage capacities to be achieved in reduced dimensions and with low production costs.
In summary, probe-storage systems envisage the use of a two-dimensional array of transducers (or probes) fixed to a common substrate and each provided with a respective read/write head. The two-dimensional array is positioned above a storage medium (e.g., made of polymeric material, ferro-electric material, phase-change material, etc.), and is relatively mobile with respect thereto.
Each probe can be actuated for interacting locally with a portion of the storage medium, for writing, reading or erasing individual information bits. In particular, the relative movement between the storage medium and the array of transducers is generated by a micromotor coupled to the storage medium.
In this connection, electrostatic micromotors are known for generating a linear movement, which are obtained with semiconductor-micromachining technologies (the so-called “MEMS technologies”). These electrostatic micromotors base their operation on a capacitive interaction between a fixed substrate (known as “stator”) and a mobile substrate that is able to move with respect to the fixed substrate (known as “rotor”, without this term implying, however, the presence of a rotary movement).
The rotor substrate is generally suspended over the stator substrate via elastic elements, at a distance of separation (gap) of, for example, a few microns. Electrostatic-interaction elements carried by the rotor substrate and stator substrate, for example, rotor and stator electrodes arranged in an appropriate way on respective facing surfaces, determine, when suitably biased, a relative movement of translation of the rotor substrate with respect to the stator substrate in a sliding direction. In particular, the stator electrodes and rotor electrodes form capacitors with plane parallel faces that are misaligned. When an appropriate biasing voltage is applied between the misaligned faces, an electrostatic interaction force is generated, which tends to bring them back into a position of mutual alignment.
In detail, the electrostatic interaction force thus generated has a useful component along the sliding direction, which determines the relative movement between the rotor substrate and the stator substrate, and also a disturbance component oriented in a direction orthogonal to the sliding direction, which tends to bring the two substrates closer to one another, generating undesirable oscillations of the rotor substrate in the orthogonal direction.
In a known way, one of the main targets in the development of electrostatic micromotors is to reduce the effects of the disturbance component of the electrostatic interaction force. For this purpose, an attempt is made, for example, to maximize the useful component with respect to the disturbance component or, equivalently, to maximize the ratio between the useful component and the disturbance component of the electrostatic interaction force. In particular, it is in any case desirable to guarantee a minimum target of stability of the rotor substrate as regards the deformations in the orthogonal direction (e.g., the deformations should be less than 100 nm, considering a thickness for the rotor substrate of approximately 400 μm). It is evident that this target of stability is extremely stringent, in particular as regards the stiffness of the elastic elements, which may need to counter the undesirable movement of the rotor substrate due to the electrostatic interaction force in the orthogonal direction.